1. Field of the Invention
The present invention generally relates to mobile communication apparatus and, more particularly, to a mobile communication apparatus for use in a cellular mobile radio communication system, capable of determining a current location of the mobile apparatus in an accommodating cell by measuring a reception level of a broadcast channel transmitted from a base station and controlling a reception level measurement process in accordance with the current location.
2. Description of the Related Art
FIG. 8 shows a mobile radio communication system according to the related art. Referring to FIG. 8, numeral 10 indicates an accommodating cell in which a mobile station 1 is located, 11 indicates a base station managing the accommodating cell 10, 12 indicates a broadcast channel such as BCCH transmitted from the base station 11, 13 indicates a common bidirectional control channel such as SACCH transmitted from the mobile station 1, 14 indicates a cell adjacent to the accommodating cell 10, 15 indicates a base station managing the adjacent cell 14, and 16 indicates a broadcast channel transmitted from the base station 15. The mobile station 1 measures a reception level with respect to the accommodating cell 10 and the adjacent cell 14 by receiving the broadcast channels 12 and 16.
The mobile station 1 measures the reception level with respect to the accommodating cell 10 and the adjacent cell 14 at a predetermined interval and sends the result of measurement with respect to the accommodating cell 10 and the adjacent cell 14 to the base station 11 of the accommodating cell 10 via the SACCH channel using burst transmission. The base station 11 determines whether a handover is necessary in accordance with the result of measurement of reception level with respect to the accommodating cell received from the mobile station 1. The process of reception level measurement and the burst transmission consume a relatively large portion of the power needed for the mobile station.
One approach to reduce power consumed in the reception level measurement and the burst transmission is disclosed in Japanese Laid-Open Patent Application No. 8-195711. FIG. 9A illustrates a first reception level measurement, FIG. 9B illustrates a second reception level measurement, and FIG. 9C illustrates a third reception level measurement. In a mobile communication system disclosed in Japanese Laid-Open Patent Application No. 8-195711, the mobile station measures the reception level of common line channels (frequencies F1-F7) transmitted from the base station of the accommodating cell and those of the adjacent cells. More specifically, as shown FIG. 9A, the first measurement is performed by alternatively measuring the reception level of the common line channel (frequency F1) of the accommodating cell and the common line channel (one of frequencies F2-F7) of an adjacent cell. The mobile station organizes the adjacent cells into a first group characterized by a relatively high reception level and a second group characterized by a relatively low reception level.
The second measurement is performed as shown in FIG. 9B by measuring the reception level of the signals with the frequencies F2, F4 and F7 corresponding to respective adjacent cells characterized by a relatively high reception level and measuring the signal with the frequency F1 corresponding to the accommodating cell. In the second measurement, the reception level measurement process is suspended during time frames used to measure the signals with the frequencies F3, F5 and F6 in the first measurement. In the third reception level measurement that follows, a reception level measurement process like that of the first reception level measurement is performed. By organizing the adjacent cells into two groups characterized by a relatively high reception level and a relatively low reception level, respectively, in the first measurement process, and by measuring the reception levels only with respect to the adjacent cells characterized by a relatively high reception level, power consumed by the mobile station is reduced.
When the mobile station reaches the neighborhood of a boundary of the accommodating cell, the reception level of the radio wave arriving from the base station of the accommodating cell drops. This causes the quality of communication between the mobile station and the base station to deteriorate. When the base station detects a drop in the reception level with respect to the accommodating cell reported by the mobile station, the base station determines that a handover is necessary. Upon determination that a handover is necessary, the most suitable destination cell is selected based on the results of measurement of reception level of the adjacent cells reported by the mobile station. When the mobile station is located near the center of the accommodating cell, the reception level is stable and the state of communication is satisfactory. Therefore, the mobile station is not required to perform the reception level measurement process and the transmission of the measurement result to the base station.
Similarly, when the mobile station is stationary in the accommodating cell or moving at an extremely slow speed, the mobile station is not required to perform the reception level measurement process and the transmission of the measurement result because the physical position with respect to the accommodating cell remains unchanged and the reception level remains substantially constant. However, the frequency of reception level measurement process should be changed depending on the speed of movement of the mobile station. For example, in order to maintain a high precision in reception level measurement in a severe fading condition occurring when the mobile station is moving at a high speed, for example, the reception level measurement process should be performed at predetermined intervals so that levels measured at a large number of sample points should be averaged so as to determine average point-by-point electric field intensity levels.
In a slight fading condition in which the mobile station is stationary or moving at a slow speed, it is not necessary to provide as many sample points as are provided in a severe fading condition since high-precision reception level measurement is possible using a smaller number of sample points in a slight fading condition. Therefore, when the mobile station is stationary or moving at a relatively low speed, power consumption can be reduced by decreasing the frequency of reception level measurement process as compared to a high-speed condition.
In the related-art reception level measurement process, the current location or movement of the mobile station in the accommodating cell is not considered, thus preventing precise reception level measurement control from being performed.